Liquid absorbing sheet and nonaqueous electrolyte battery pack

ABSTRACT

A liquid absorbing sheet includes a liquid absorbing resin layer that can effectively absorb nonaqueous electrolyte solutions used in nonaqueous electrolyte secondary cells that make nonaqueous electrolyte battery packs (in particular, lithium ion-based nonaqueous secondary battery packs). The liquid absorbing resin layer is obtained by irradiating UV-rays onto a monomer composition to polymerize the monomer composition, the monomer composition containing: a monofunctional monomer component containing a monofunctional monomer capable of forming a homopolymer that is soluble in a nonaqueous solvent used in a nonaqueous electrolyte secondary battery; and a polyfunctional monomer component.

TECHNICAL FIELD

The present invention relates to a liquid absorbing sheet for absorbingan electrolyte solution when such a solution leaks from a nonaqueouselectrolyte battery cell encased in a nonaqueous electrolyte batterypack. The present invention also relates to a nonaqueous electrolytebattery pack that uses such a liquid absorbing sheet.

BACKGROUND ART

Battery packs are widely used that have a plurality of primary orsecondary battery cells, a circuit board, and a battery case encasingthese components. When an electrolyte solution leaks from any of thebattery cells, it can corrode the wiring of the circuit board, resultingin a conduction failure or short circuit. To prevent such corrosion andshort circuits when leakage of the electrolyte solution occurs, a liquidabsorbing element containing an absorbent capable of absorbing theelectrolyte solution is arranged in the battery pack adjacent to or inthe vicinity of the battery cell (Japanese Patent Application Laid-OpenNo. 2001-351588). Various polymer materials are used as the absorbent,including adsorbents, gelling agents and self-swelling agents. Amongspecific examples of the absorbents described are polyacrylate-basedwater-absorbing resins, starch/graft copolymer-based water-absorbingresins, polyvinyl alcohol-based water-absorbing resins,polyacrylamide-based water-absorbing resins, isobutyrene-maleic acidcopolymer-based water-absorbing resins, long chain alkyl acrylatecrosslinked polymers, and polynorbornens.

One drawback of these absorbents is that they cannot effectively absorbpropylene carbonate, dimethyl carbonate and other carbonate-basedsolvents that are widely used in nonaqueous electrolyte battery packs, atype of batteries that have become increasingly used in recent years.Specifically, these solvents are used in nonaqueous electrolytesecondary cells that make lithium ion-based nonaqueous electrolytesecondary battery packs.

The present invention addresses the above-described problem and to thatend, it is an objective of the present invention to provide a liquidabsorbing sheet capable of effectively absorbing the nonaqueouselectrolyte solution used in nonaqueous electrolyte secondary cells thatmake nonaqueous electrolyte battery packs (in particular, lithiumion-based nonaqueous secondary battery packs). It is another objectiveof the present invention to provide a battery pack equipped with anelectrolyte-absorbing element made of such a liquid absorbing sheet.

DISCLOSURE OF THE INVENTION

In the course of our study, the present inventors have found that aresin layer obtained in a particular manner can absorb and retainsignificant amounts of an electrolyte solution, the finding leading tothe present invention. Specifically, this resin layer is obtained byirradiating UV-rays onto a particular monomer composition to polymerizethe composition and thereby make a sheet. This monomer compositioncontains the following components: a monofunctional monomer component(A) comprising a monofunctional monomer (a) capable of forming ahomopolymer that is soluble in nonaqueous solvents used in nonaqueouselectrolyte secondary batteries; and a polyfunctional monomer component(B).

Specifically, the present invention provides a liquid absorbing sheetcomprising a liquid-absorbing resin layer, wherein the liquid-absorbingresin layer is obtained by irradiating UV-rays onto a monomercomposition to polymerize the monomer composition, the monomercomposition containing:

a monofunctional monomer component (A) containing a monofunctionalmonomer (a) capable of forming a homopolymer that is soluble in anonaqueous solvent used in a nonaqueous electrolyte secondary battery;and

a polyfunctional monomer component (B).

The present invention also provides a nonaqueous electrolyte batterypack having a nonaqueous electrolyte battery cell, a circuit board, anelectrolyte-absorbing element for absorbing an electrolyte solution inthe event of electrolyte leakage from the nonaqueous electrolyte batterycell, and a battery case encasing all of the above components, whereinthe electrolyte-absorbing element is formed of the above-describedliquid absorbing sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C are each a cross-sectional view showing a liquidabsorbing sheet of the present invention.

FIG. 2 is a perspective view showing a nonaqueous electrolyte batterypack of the present invention.

FIG. 3 is a perspective view showing another nonaqueous electrolytebattery pack of the present invention.

FIG. 4 is a diagram illustrating an electrolyte absorption testconducted using a model battery pack.

BEST MODE FOR CARRYING OUT THE INVENTION

First the liquid absorbing sheet of the present invention will bedescribed below in more detail.

The liquid absorbing sheet of the present invention can be provided indifferent forms. For example, the liquid absorbing sheet may be providedas an independent sheet formed entirely of a liquid absorbing resinlayer 1 (FIG. 1A), or it may comprise a substrate 2 with the liquidabsorbing resin layer 1 formed on one side (FIG. 1B). Alternatively, theliquid absorbing sheet may include an adhesive layer 3 formed over theliquid absorbing resin layer 1 (FIG. 1C). The liquid absorbing sheetwith the configuration shown in FIG. 1C can be easily secured in abattery case. The adhesive layer 3 may be formed of any conventionaladhesive. The liquid absorbing sheet of the present invention may notinclude the substrate: it may be the adhesive layer formed on one sideof the liquid absorbing resin layer (the configuration shown in FIG. 1Cwithout the substrate 2).

While the adhesive layer 3 may be formed of any conventional adhesive,it is preferably formed of a liquid absorbing resin layer that itselfshows adhesion. Whether a liquid absorbing resin exhibits adhesion ornot depends on its composition, as will described later. By using theadhesive layer 3 that serves both as an adhesive and as a liquidabsorber, the amount of the liquid that the liquid absorbing sheet canabsorb can be increased as compared to a liquid absorbing sheet using asimple adhesive layer that does not absorb a liquid. We will describethis later in the description.

Although the substrate 2 for use in the liquid absorbing sheet of thepresent invention may be a resin film that is impermeable to electrolytesolutions (such as a plastic film made of polypropylene or othermaterials), it may be a material that can absorb and retain thenonaqueous solvent, including nonwoven fabric or synthetic paper formedof plastic fibers such as polypropylene, or paper. The substrate made ofsuch a nonwoven fabric can absorb the nonaqueous solvent at an increasedrate.

The liquid absorbing resin layer 1 to form the liquid absorbing sheet ofthe present invention may be a polymer film obtained by irradiatingUV-rays onto a particular monomer composition to polymerize thecomposition and thereby make a sheet. This monomer composition containsthe following components: a film-forming, monofunctional monomercomponent (A) comprising a monofunctional monomer (a) capable of forminga homopolymer that is soluble in nonaqueous solvents used in nonaqueouselectrolyte secondary batteries; and a crosslinkable, polyfunctionalmonomer component (B).

The monofunctional monomer component (A) for use in the presentinvention must be a monofunctional monomer (a) that can form ahomopolymer soluble in nonaqueous solvents used in nonaqueouselectrolyte secondary batteries. This is because, if the monofunctionalmonomer component (A) is composed solely of a monofunctional monomerthat forms a homopolymer insoluble in such nonaqueous solvents, theability of the resulting resin layer to absorb the nonaqueous solventbecomes insufficient. By saying “a homopolymer is soluble in anonaqueous solvent,” it is meant that the weight of the homopolymer isdecreased by at least 10% when 1 part by weight of the homopolymer isimmersed in 30 parts by weight of a nonaqueous solvent for 24 hours atroom temperature (approx. 23° C.). The nonaqueous solvent is especially,a mixed solvent containing at least one of dimethyl carbonate, propylenecarbonate, and ethylene carbonate, which will be described later in thedescription. The mixture preferably contains equal volumes of therespective solvents. The decrease in weight can be determined bycomparing the dry weight of the homopolymer measured after thehomopolymer has been immersed in the nonaqueous solvent and then pulledout of the solvent, with the weight measured prior to the immersionperiod. A 100% decrease in weight means that the homopolymer has beencompletely dissolved in the solvent.

The monofunctional monomer (a) is such that the difference in thesolubility parameter value (i.e., SP value (J/cm³)^(1/2)) between themonofunctional monomer and a given nonaqueous solvent used in anonaqueous electrolyte secondary battery to which to apply the liquidabsorbing sheet preferably falls within the range of −1.0 to 8.0 and,more preferably, within the range of 2.0 to 6.5. If this differencefalls outside the specified range, the homopolymer becomes substantiallyinsoluble in the nonaqueous solvent, resulting in an insufficientability of the resulting resin layer to absorb the solvent.

The solubility parameter is determined by the Fedors equation shownbelow (See, R. F. Fedors, Polym. Eng. Sci., 14(2), p 147, p 472 (1947)).The solubility of the monofunctional monomer in the present inventionwas determined for polymerized repeating units. In the following Fedorsequation, ‘σ’ indicates the solubility parameter, ‘V’ indicates themolar volume (cm³/mol) and ‘E_(coh)’ indicates the binding energy(J/mol):σ=(ΣE _(coh) /V)^(1/2)

The monofunctional monomer (a) may be a mixture of monofunctionalmonomers. Given that such a monomer mixture contains n mols of a monomer(a1) with a solubility parameter value of SP1 and m mols of a monomer(a2) with a solubility parameter value of SP2, then the solubilityparameter value of the monomer mixture (SP_((monomer)mix)) is determinedby the following equation, which is equally applied to determine thesolubility parameter for monomer mixtures containing three or moremonofunctional monomers:SP _((monomer)mix)=(SP1×n+SP2×m)/(n+m)

Specific examples of such monofunctional monomers (a) include imideacrylate (SP=27.6), N-vinyl-2-pyrrolidone (SP=26.2), acryloyl morpholine(SP=25), benzyl acrylate (SP=22.9), phenoxyethyl acrylate (SP=22.6),N,N-diethylacrylamide (SP=20.6), methoxypolyethylene glycol acrylate(ethylene oxide-added mol number (n)=9, SP=19.6), methoxypolyethyleneglycol acrylate (ethylene oxide-added mol number (n)=3, SP=20.1),tetrahydrofurfuryl acrylate (SP=23), and phenoxypolyethylene glycolacrylate (ethylene oxide-added mol number (n)=6, SP=20.7). Of these,benzyl acrylate, N-vinyl-2-pyrrolidone, tetrahydrofurfuryl acrylate, andacryloyl morpholine are preferred because of the high absorbability ofthe film for the electrolyte solution. These monomers may be used incombination of two or more. For the purpose of optimizing the balancebetween the absorbability of the film for the electrolyte solution andthe hardness of the film, it is particularly preferred to use benzylacrylate in conjunction with acryloyl morpholine. In such a case, theratio of benzyl acrylate to acryloyl morpholine (by weight) ispreferably from 30/70 to 70/30. Acryloyl morpholine, when present inexcess, improves the absorbability of the film for the electrolytesolution but makes the film hard and susceptible to cracking.

The solubility parameter value of the nonaqueous solvent is preferablyin the range of 17 to 28 and, more preferably, in the range of 18 to 23.If the nonaqueous solvent has a solubility parameter value that fallsoutside the specified range, then the solvent, when used in a lithiumbattery, can cause a decrease in the cycle performance of the battery.

Among the nonaqueous solvents for use in the present invention arecarbonates, including dimethyl carbonate (SP=17.4), propylene carbonate(SP=20.8), and ethylene carbonate (SP=22.5). These carbonates may beused either individually or in combination of two or more. Oneparticularly preferred nonaqueous solvent is a mixed solvent(SP_((solvent)mix)=20.2) containing equal volumes of dimethyl carbonate,propylene carbonate, and ethylene carbonate. Using the solubilityparameter and the amount (in the number of mols) of each of thenonaqueous solvents used in the mixture, the solubility parameter of themixed nonaqueous solvent can be determined in the same manner as in thedetermination of the solubility parameter of multiple monofunctionalmonomers used in combination.

The homopolymer of the monofunctional monomer (a) is preferably obtainedby adding 0.1 to 5 parts by weight of a UV polymerization initiator(e.g., 2-hydroxy-2-methyl-1-phenylpropane-1-one, bis-acyl phosphineoxide, benzophenone, and 2-methylthioxanthone) to 100 parts by weight ofthe monofunctional monomer (a), and irradiating the mixture with UV rayswith a wavelength of 250 to 350 nm at an energy density of 100 to 2000mJ/cm² to polymerize the monomer.

According to the present invention, the monofunctional monomer component(A) preferably contains the monofunctional monomer (a) in an amount ofat least 20 mol %. If too little of the monomer (a) is present, theamount of the nonaqueous solvent that can be absorbed by the liquidabsorbing sheet may be reduced.

As long as the advantages of the present invention are not affected,other monofunctional monomers, such as hydroxyethyl acrylate (SP=29.6),acrylic acid (SP=28.7), 2-ethylhexyl acrylate (SP=18.9), and laurylacrylate (SP=18.7), may be added to the monofunctional monomer component(A).

The polyfunctional monomer component (B) for use in the presentinvention serves to introduce crosslinks in the liquid absorbing resinlayer 1 and is preferably a monomer having two or more acrylateresidues. Examples of such monomers are hydroxyl pivalic acid neopentylglycol diacrylate, polyethylene glycol diacrylate (ethylene oxide-addedmol number (n)=14), bisphenol A diacrylate, phenyl glycidyl etheracrylate, and hexamethylene diisocyanate urethane prepolymer.

The amount of the polyfunctional monomer component (B) added to themonomer composition is such that the crosslink density preferably fallsin the range of 0.0001 to 0.17 and, more preferably, in the range of0.001 to 0.1. Too small an amount of the polyfunctional monomercomponent (B) may make it difficult for the liquid absorbing resin layer1 to retain its shape, whereas too large an amount of the polyfunctionalmonomer component (B) may lead to an insufficient ability of the liquidabsorbing resin layer 1 to absorb the nonaqueous solvent.

Given that ‘a’ indicates the number of the functional groups borne by asingle molecule of the polyfunctional monomer, ‘b’ indicates the numberof mols of the polyfunctional monomer present in the monomercomposition, and ‘c’ indicates the number of mols of the monofunctionalmonomer present in the monomer composition, the crosslink density can bedefined by the following equation:Crosslink density=a×b/(b+c).

The first configuration of the liquid absorbing sheet of the presentinvention as depicted in FIG. 1A can be obtained by coating a peelablefilm, such as polyethylene terephthalate film, with the above-describedmonomer composition containing the monofunctional monomer component (A)and the polyfunctional monomer component (B), irradiating the coatedfilm with UV rays to polymerize the composition and thereby form asheet, and peeling the sheet from the peelable film. The secondconfiguration as depicted in FIG. 1B can be obtained either by coating anonwoven fabric with the monomer composition and polymerizing thecomposition, or by laminating a nonwoven fabric onto the firstconstruction of FIG. 1A. The third construction as depicted in FIG. 1Ccan be obtained by further applying or laminating an adhesive over theliquid absorbing resin layer of the second configuration of FIG. 1B.

By using a peelable sheet embossed with surface patterns, the surfacearea of the liquid absorbing resin layer can be increased, resulting inan increase in the absorption rate of the liquid absorbing sheet.

The monomer composition containing the monofunctional monomer component(A) and the polyfunctional monomer component (B) may be directly appliedto the inner surface of a battery case 21 as shown in FIG. 2, ratherthan onto the peelable film, such as polyethylene terephthalate film.Then, by irradiating with UV rays, the composition can be polymerized toform a sheet where it has been applied.

The monomer composition can be applied to the peelable sheet or nonwovenfabric by any suitable conventional technique, such as roll coatertechnique. The UV polymerization is typically carried out at 15 to 25°C. while irradiating UV rays with a wavelength of 250 to 350 nm at anenergy density of 100 to 2000 mJ/cm².

When the liquid absorbing resin layer itself shows adhesion (e.g., whenthe monofunctional monomer used in the component (A) istetrahydrofurfuryl acrylate (SP=23), benzyl acrylate (SP=22.9),phenoxyethyl acrylate (SP=22.6), phenoxypolyethylene glycol acrylate(ethylene oxide-added mol number (n)=6, SP=20.7), or methoxypolyethyleneglycol acrylate (ethylene oxide-added mol number (n)=3, SP=20.1)), theconstruction of FIG. 1A or FIG. 1B can be directly attached to thebattery pack without providing the adhesive layer. In addition, theliquid absorbing resin layer can be attached to the substrate by using ahand roller at room temperature, rather than by thermal lamination (See,FIG. 1B). Considering the fact that the leakage in many cases occurs atthe cathode of the cylindrical batteries, the liquid absorbing sheet ispreferably shaped as a doughnut-like shape in its plan view so that itcan be applied about the cathode terminal.

According to the present invention, a flame retardant (e.g.,phosphate-based liquid flame retardant, aluminum hydroxide, and melaminecyanurate) may be further added to the liquid absorbing resin layer 1 inthe liquid absorbing sheet of the present invention. This imparts aflame retardancy to the liquid absorbing sheet. Specifically, when aphosphate-based liquid flame retardant is used as the flame retardant ofthe present invention, it can impart to the liquid absorbing resin layer1 a flame retardancy of grade V-0, V-1, or V-2 according to the UL-94standard. Furthermore, the phosphate-based flame retardant can impart ahigh adhesion to the liquid absorbing resin layer 1 since the agentremains a liquid under atmospheric pressure, typically at −13° C. to250° C., and preferably at room temperature. The liquid absorbing resinlayer 1 that shows adhesion can be directly attached to the nonaqueouselectrolyte secondary battery pack and, thus, can eliminate the need toprovide an additional adhesive layer, so that the thickness of theliquid absorbing resin layer 1 does not have to be reduced by an amountcorresponding to the thickness of the adhesive layer. Accordingly, theamount of the nonaqueous solvent that can be absorbed by the liquidabsorbing sheet can be maintained. Moreover, we have unexpectedly foundthat when the phosphate-based liquid flame retardant is used as theflame retardant of the present invention, the insulation resistance ofthe liquid absorbing resin layer 1 can be preferably maintained above1×10¹²Ω even after the liquid absorbing sheet has been subjected to awet heat aging process (e.g., place the liquid absorbing sheet at atemperature of 40° C. under a humidity of 90% RH for 96 hours).

Preferred examples of the phosphate-based liquid flame retardant for usein the present invention include bisphenol A bis(diphenyl)phosphate,hydroquinol bis(diphenyl)phosphate, phenyl dixylenyl phosphate,tricresyl phosphate, cresyl diphenyl phosphate, trixylenyl phosphate,xylenyl diphenyl phosphate, resorcinol bis(diphenyl)phosphate, and2-ethylhexyl diphenyl phosphate. Of these, bisphenol Abis(diphenyl)phosphate, hydroquinol bis(diphenyl)phosphate and phenyldixylenyl phosphate are preferred since they can impart high adhesion tothe liquid absorbing resin layer 1.

The amount of the phosphate-based liquid flame retardant is preferablyin the range of 70 to 200 parts by weight and, more preferably, in therange of 100 to 150 parts by weight with respect to 100 parts by weightof the monofunctional monomer component (A) and the polyfunctionalmonomer component (B) combined. If too little or too much of the flameretardant is added, desired flame retardancy cannot be achieved.

The liquid absorbing sheet of the present invention is suitable as anelectrolyte-absorbing element used in an nonaqueous electrolyte batterypack that consists of a battery case encasing nonaqueous electrolytebattery cells, a circuit board, and the electrolyte-absorbing element.The liquid absorbing sheet serves to absorb the electrolyte solutionshould leakage occur from the battery cell. One example of such abattery pack is shown in FIG. 2. The battery pack includes a batterycase 21, which encases a circuit board 22 and a plurality of nonaqueouselectrolyte battery cells 23 arranged on the circuit board 22. A liquidabsorbing sheet 26 as described above with reference to FIG. 1A isarranged between the circuit board 22 and the nonaqueous electrolytebattery cells 23 for absorbing the electrolyte solution should leakageoccur from any of the nonaqueous electrolyte battery cells. A metal lead24 connects the circuit board 22 with each of the nonaqueous electrolytebattery cells 23. The metal lead 24 is also connected to externalterminals 25. As shown in FIG. 3, a liquid absorbing sheet 27 asdescribed above with reference to FIG. 1C may be arranged on top of thenonaqueous electrolyte battery cells 23 with its substrate facing thenonaqueous electrolyte battery cells 23.

Although the battery cases of the respective nonaqueous electrolytebattery packs shown in FIGS. 2 and 3 are rectangular parallelepiped withcylindrical battery cells, the shape and arrangement of the battery caseand the battery cells, as well as the type of the battery cells, are notlimited to those shown in the figures and may vary depending on theintended purposes of the batteries.

As set forth, the nonaqueous electrolyte battery packs of the presentinvention include the liquid absorbing sheet, which serves as an elementfor absorbing electrolyte solution should leakage of the solution occurfrom the battery cells. The liquid absorbing sheet achieves thisfunction by having a liquid absorbing resin layer that can effectivelyabsorb and retain the nonaqueous electrolyte solution. This liquidabsorbing resin layer is formed of a monomer composition that contains amonofunctional monomer component (A), which comprises a monofunctionalmonomer (a) capable of forming a homopolymer soluble in nonaqueoussolvents used in nonaqueous electrolyte secondary batteries, and apolyfunctional monomer component (B). The liquid absorbing sheet, whenused in nonaqueous electrolyte battery packs, significantly reduces theoccurrence of corrosion and short circuits of the circuit board in theevent of leakage of the nonaqueous electrolyte solution.

EXAMPLES

The present invention will now be described in detail with reference toexamples.

Reference Example

Homopolymers formed of different monofunctional monomers were examinedfor their solubility in a nonaqueous solvent used in nonaqueouselectrolyte secondary batteries.

Specifically, 1 part by weight of a photopolymerization initiator(2-hydroxy-2-methyl-1-phenylpropane-1-one (D1173, Ciba SpecialtyChemicals)) was added to 100 parts by weight of a monofunctionalmonomer. Using a roll coater, the mixture was applied to a polyethyleneterephthalate film and was irradiated with a UV-ray with a wavelength of365 nm. The UV-ray was shone at an energy density of 2000 mJ/cm². Thiscaused the composition to polymerize and thereby form a polymer film. 1part by weight of the resulting film was immersed in 300 parts by weightof a mixture containing equal volumes of dimethylcarbonate, propylenecarbonate, and ethylene carbonate (SP_((solvent)mix)=20.2) at 23° C. for24 hours. Subsequently, the mixture was filtrated and the solid productremaining on the filter was dried at 100° C. for 1 hour. The solubility(wt %) of the dried product was determined by the equation below withthe results shown in Table 1. In the following equation, W₁ indicatesthe weight of the film prior to the immersion period and W₂ indicatesthe weight of the dried solid product:Solubility=((W ₁ −W ₂)/W ₁)×100. TABLE 1 SP Solubility Monofunctionalmonomer value ΔSP (wt %) Hydroxyethyl acrylate 29.6 9.4 3 Acrylic acid28.7 8.5 5 Imide acrylate 27.6 7.4 100 N-vinyl-2-pyrrolidone 26.2 6.0100 Acryloyl morpholine 25 4.8 100 Benzyl acrylate 22.9 2.7 100Phenoxyethyl acrylate 22.6 2.4 85 N,N-diethylacrylamide 20.6 0.4 100Methoxypolyethylene glycol 19.6 −0.6 100 acrylate (n = 9)Methoxypolyethylene glycol 20.1 −0.1 100 acrylate (n = 3)Phenoxypolyethylene glycol 20.7 0.5 100 acrylate (n = 6)Tetrahydrofurfuryl acrylate 23.0 2.8 100 2-ethylhexyl acrylate 18.9 1.33

Examples 1 through 6 and Comparative Examples 1 through 3

One of the different monofunctional monomers shown in Tables 2 and 3,hydroxyl pivalic acid neopentyl glycol diacrylate to serve as apolyfunctional monomer and 2-hydroxy-2-methyl-1-phenylpropane-1-one toserve as a polymerization initiator were mixed together in theproportions shown in Tables 2 and 3. Using a roll coater, the mixturewas applied to a polyethylene terephthalate film and was irradiated witha UV-ray with a wavelength of 365 nm. The UV-ray was shone at an energydensity of 2000 mJ/cm² to cause the composition to polymerize andthereby form a polymer film. The polymer film was peeled off from thepolyethylene terephthalate film to give a single-layered liquidabsorbing sheet.

The resulting liquid absorbing sheet was immersed in a large volume of amixed solvent containing equal volumes of dimethylcarbonate, propylenecarbonate, and ethylene carbonate (SP_((solvent)mix)=20.2) at 23° C.After 2 hours, the appearance of the liquid absorbing resin layer wasvisually examined. The liquid absorbing sheet was pulled out of themixed solvent and the solvent remaining on the surface was immediatelywiped off. The sheet was weighed and its swell ratio was determined. Theresults are shown in Table 2. Each liquid absorbing sheet was alsoexamined for the SP value of the monofunctional monomer(SP_((monomer))), the difference in the SP value between themonofunctional monomer and the solvent (ΔSP value), the density ofcrosslinks in the liquid absorbing resin layer of the liquid absorbingsheet and the state after the immersion period. The results are alsoshown in Tables 2 and 3. TABLE 2 Examples (wt parts) Components 1 2 3 45 6 Benzyl acrylate 100 100 100 — 30 100 N-vinyl-2- — — — 100 — —pyrrolidone Acrylic acid — — — — 70 — Polyfunctional 1 0.1 10 1 1 20monomer Photopolymerization 1 1 1 1 1 1 initiator SP_((monomer)) 22.922.9 22.9 26.2 27.8 22.9 ΔSP 2.7 2.7 2.7 6.0 7.6 2.7 Crosslink density0.010 0.001 0.091 0.007 0.005 0.167 Swell ratio (times) 10 10 3 10 3 1.6State after Film Gel Film Film Film Film immersion

TABLE 3 Comparative Example (wt parts) 1 2 3 Benzyl acrylate 100 — —Acrylic acid — 100 — 2-ethylhexyl acrylate — — 100 Polyfunctionalmonomer — 1 1 Photopolymerization 1 1 1 initiator SP_((monomer)) 22.928.7 18.9 ΔSP 2.7 8.5 −1.3 Crosslink density 0.000 0.005 0.005 Swellratio (times) Dissolved 1.1 1.1 State after immersion Liquid Film Film

As shown by the results of Tables 2 and 3, the liquid absorbing sheet ofExample 1 swelled 10 times and still retained its film-like shape afterit had swollen by absorbing the nonaqueous solvent. This implies thatthis liquid absorbing sheet can serve as an effective liquid absorbingelement for absorbing the electrolyte solution should leakage of thesolution occur from the nonaqueous electrolyte battery cells.

The results also indicate that the liquid absorbing sheets of Examples 2through 6 each exhibits a good performance that allows the practical useof the liquid absorbing sheet as an element for absorbing theelectrolyte solution. Nonetheless, the results of Example 2 indicatethat the liquid absorbing sheet with a decreased crosslink density tendsto become a gel-like material, rather than retaining its film-shape,after absorbing the nonaqueous solvent and swelling. In contrast, theresults of Examples 3 and 6 show that an increased crosslink densitytends to result in a decreased swell ratio. The results of Example 4indicate that a good result can be achieved by using, rather than benzylacrylate, N-vinyl-2-pyrrolidone as the monofunctional monomer. Theresults of Example 5 indicate that when a monofunctional monomer whosehomopolymer is insoluble in the nonaqueous solvent is used together, theswell ratio of the liquid absorbing sheet tends to decrease.

On the other hand, the results of Comparative Example 1 indicate thatthe polyfunctional monomer-free liquid absorbing resin layer dissolvesin the nonaqueous solvent and thus cannot be used as the element forabsorbing the electrolyte solution. The results of Comparative Example 2indicate that when the monofunctional monomer has too large a SP valueso that its homopolymer is substantially insoluble in the nonaqueoussolvent, the swell ratio of the liquid absorbing sheet becomes toosmall, making the liquid absorbing sheet unsuitable for use as theelement for absorbing the electrolyte solution. The results ofComparative Example 3 indicate that when the monofunctional monomer hastoo small a SP value so that its homopolymer is again substantiallyinsoluble in the nonaqueous solvent, the swell ratio of the liquidabsorbing sheet becomes too small, making the liquid absorbing sheetunsuitable for use as the element for absorbing the electrolytesolution.

Example 7 Test for the Ability to Absorb an Electrolyte Solution Using aSimulated Battery Pack

As shown in FIG. 4, a 7.0 cm (1)×7.9 cm (w)×2.3 cm (h) ABS resin box 41was obtained. A 6.5 cm (1)×6.5 cm (w) 100 μm-thick liquid absorbingsheet 42 prepared according to Example 1 was stuck to the bottom of thebox with a commercially available adhesive. Three lithium ion batteries43 were placed on the liquid absorbing sheet 42, and a glass epoxysubstrate 44 as a circuit board was placed adjacent to the batteries.

A hole h was drilled through the side wall of one of the batteries 43that was in the middle of the three. The electrolyte solution leakingfrom the hole was allowed to be absorbed by the liquid absorbing sheet.After drilling of the hole h, the batteries were left for one day andnight and the inside of the battery pack was observed. It turned outthat the glass epoxy substrate was not wet. In addition, the decrease inweight of the battery with the hole h drilled in it was 2.5 g, which wasequal to the increase in weight of the liquid absorbing sheet. Theseobservations suggest that the leaked electrolyte solution was entirelyabsorbed by the liquid absorbing sheet.

Examples 8 through 15

One of the different monofunctional monomers shown in Table 4,polyethylene glycol diacrylate to serve as a polyfunctional monomer(ethylene glycol-added mol number=14, 14EG-A, Kyoeisha Chemical) and2-hydroxy-2-methyl-1-phenylpropane-1-one to serve as a polymerizationinitiator were mixed together in the proportions shown in Table 4. Usinga roll coater, the mixture was applied to a polyethylene terephthalatefilm and was irradiated with a UV-ray with a wavelength of 365 nm. TheUV-ray was shone at an energy density of 2000 mJ/cm² to cause thecomposition to polymerize and thereby form a polymer film. The polymerfilm was then peeled off from the polyethylene terephthalate film togive a single-layered liquid absorbing sheet (210 g/m²).

Meanwhile, an electrolyte solution was prepared by adding lithiumphosphate hexafluoride to serve as an electrolyte to a mixed solventcontaining equal volumes of dimethylcarbonate, propylene carbonate, andethylene carbonate (SP_((solvent)mix)=20.2). 0.2 ml of this electrolytesolution was added dropwise onto 0.03 g of the liquid absorbing sheetand the time it took for the solution to be completely absorbed by theliquid absorbing sheet was visually measured. The liquid absorbing sheetwas also immersed in a large volume of the electrolyte solution at 23°C. After 3 hours, the appearance of the liquid absorbing resin layer wasvisually examined. The liquid absorbing sheet was pulled out of themixed solvent and the solvent remaining on the surface was immediatelywiped off. The sheet was weighed and its swell ratio was determined.Furthermore, the liquid absorbing sheet was heated in a wet heat oven(40° C., 90% RH, 96 hrs) and was examined for the degree of swelling.The results are shown in Table 5.

In addition, a 5 cm wide strip of a polypropylene nonwoven fabric(Unitika) was laminated onto either side of the liquid absorbing sheetusing the hand roller technique (23° C.) or the thermal laminationtechnique (80° C.). The adhesion strength was then determined on atensile tester (TENSILON, Orientech) used in T-peel mode. Also, a 3 cmwide strip of the liquid absorbing sheet was laminated onto an Nisurface using the hand roller technique (23° C.) or the thermallamination technique (80° C.). The adhesion strength was then determinedon a tensile tester (TENSILON, Orientech) used in T-peel mode. Theresults are shown in Table 5.

Each liquid absorbing sheet was also examined for the SP value of themonofunctional monomer (SP_((monomer))), the difference in the SP valuebetween the monofunctional monomer and the solvent (ΔSP value), thedensity of crosslinks in the liquid absorbing resin layer of the liquidabsorbing sheet and the state after the immersion period. The resultsare also shown in Table 5. TABLE 4 Examples (wt parts) Components 8 9 1011 12 13 14 15 Tetrahydrofurfuryl 100 — — — — — — — acrylate Benzylacrylate — 100 — — — — — — Phenoxyethyl acrylate — — 100 — — — — —Phenoxypolyethylene — — — 100 — — — — glycol acrylate (n = 6)Methoxypolyethylene — — — — 100 — — — glycol acrylate (n = 3)Methoxypolyethylene — — — — — 100 — — glycol acrylate (n = 9) Acryloylmorpholine — — — — — — 100 — N,N-diethylacrylamide — — — — — — — 100Polyfunctional 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 monomerPhotopolymerization 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 initiator

TABLE 5 Examples Components 8 9 10 11 12 13 14 15 (Degree of swelling)W/O wet heat process Dropwise addition (min) 30 100 120 40 15 15 120<  180 Immersed (times) 9.0 9.4 9.6 8.2 8.6 9.0   2.3 18 (Degree ofswelling) With wet heat process Dropwise addition (min) 30 130 160 60 2025 120<   200 Immersed (times) 9.0 9.2 9.0 8.1 8.5 8.5   2.0 17 Adhesionstrength to nonwoven fabric Hand roller (kg/5 cm) 0.4 0.03 0.03 0.1 0.030 0 0 Thermal lamination (kg/5 cm) 0.7 0.03 0.03 0.1 0.03 0 0 0.3Adhesion strength to Ni surface Hand roller (kg/3 cm) 0.3 0.02 0.02 0.080.03 0 0 0 SP_((monomer)) 23 22.9 22.6 20.7 20.1 19.6 25  20.6 ΔSP 2.82.7 2.4 0.5 −0.1 −0.6   4.8 0.4 Crosslink density 0.002 0.002 0.0030.004 0.003 0.006    0.002 0.002 State after immersion Film Film FilmFilm Film Film Film Film

As shown by the results of Table 5, the liquid absorbing sheets ofExamples 8 through 13 each swelled approximately 8 to 9 times after theimmersion period and still retained the film-like shape after they hadswollen by absorbing the nonaqueous solvent. This implies that each ofthese liquid absorbing sheets, while exhibiting varying absorption ratesupon dropwise addition of a predetermined amount of the nonaqueoussolvent, can serve as an effective liquid absorbing element forabsorbing the electrolyte solution should leakage of the solution occurfrom the nonaqueous electrolyte battery cells.

Although the liquid absorbing sheet of Example 14 swelled at a lessswell ratio than the liquid absorbing sheets of Examples 8 through 13,the sheet exhibited a good performance that would allow the practicaluse of the liquid absorbing sheet as an element for absorbing theelectrolyte solution.

While the liquid absorbing sheet of Example 15 showed a slowerabsorption rate than the liquid absorbing sheets of Examples 8 through13, it had a good swell ratio.

The liquid absorbing sheets of Examples 8 through 12 each had an liquidabsorbing resin layer that itself showed adhesion, so that they did notrequire a separate adhesion layer. In particular, the liquid absorbingresin layer of the liquid absorbing sheet of Example 8 showed superioradhesion.

Example 16 (Experiments a Through e), Comparative Example 4 (Experimentsf Through h), and Comparative Example 5

The following components were mixed together in the proportions shown inTables 1 and 2: one of the different monofunctional monomers shown inTables 6 and 7, urethane acrylate (AH600, Kyoeisha) to serve as apolyfunctional monomer, 2-hydroxy-2-methyl-1-phenylpropane-1-one (D1173,Ciba Specialty Chemicals) to serve as a polymerization initiator and aphosphate-based liquid flame retardant or an ammoniumpolyphosphate-based solid flame retardant. Using a roll coater, themixture was applied to a polyethylene terephthalate film and wasirradiated with a UV-ray with a wavelength of 365 nm. The UV-ray wasshone at an energy density of 2000 mJ/cm² to cause the composition topolymerize and thereby form a polymer film. A flame-retardant nonwovenfabric (Japan Vilene) was then laminated over the polymer film and thepolyethylene terephthalate film was removed to obtain a double-layeredliquid absorbing sheet.

Each of the liquid absorbing sheets so obtained was tested for the flameretardancy, adhesion, swell ratio upon absorption of the electrolytesolution and insulating performance. The tests were conducted in thefollowing manner.

<Flame Retardancy>

The flame retardancy of each liquid absorbing sheet was evaluatedaccording to the UL-94 standard. The results are shown in Tables 6 and7. The flame retardancy of grade V-0, V-1, or V-2 indicates that theabsorbance sheet has sufficient flame retardancy for practical use.

<Adhesion>

A 5 cm wide strip of a polypropylene nonwoven fabric (Japan Vilene) waslaminated onto the liquid absorbing sheet over the liquid absorbingresin layer using the hand roller technique (23° C.). The adhesionstrength was then determined on a tensile tester (TENSILON, Orientech)used in T-peel mode. The results are shown in Tables 6 and 7.

<Swell Ratio>

An electrolyte solution was prepared by adding lithium phosphatehexafluoride to serve as an electrolyte to a mixed solvent containingequal volumes of dimethylcarbonate, propylene carbonate and ethylenecarbonate. The lithium phosphate hexafluoride was added to aconcentration of 1 mol/l. The liquid absorbing sheet was immersed in theelectrolyte solution at 23° C. After 3 hours, the liquid absorbing sheetwas pulled out of the solution and the solution remaining on the surfacewas immediately wiped off. The sheet was weighed and its swell ratio wasdetermined. The results are shown in Tables 6 and 7.

<Insulating Performance>

The insulation resistance (Ω) of the liquid absorbing resin layer ofeach liquid absorbing sheet was determined before and after the wet heatprocess. The results are shown in Tables 6 and 7. TABLE 6 Example 16 (Wtparts) Experiment Experiment Experiment Exeperiment ExperimentComponents a b c d e Benzyl acrylate 70 70 70 70 70 Acryloyl 30 30 30 3030 morpholine Polyfunctional 0.5 0.5 0.5 0.5 0.5 monomerPhotopolymerization 0.5 0.5 0.5 0.5 0.5 initiator Bisphenol A 70 100 150200 — bis(diphenyl)phosphate Hydroquinol — — — — 100bis(diphenyl)phosphate Flame V-0 V-0 V-0 V-2 V-2 retardancy(UL-94)Adhesion (kgf/5 cm) 0.4 0.6 0.5 0.5 0.7 Swell Ratio (Times) 9 8 6 6 7Insulating 4 × 10¹² 4 × 10¹² 3 × 10¹² 3 × 10¹² 1 × 10¹⁰ performance (Ω)(before wet heat process) Insulating 1 × 10¹² 1 × 10¹² 1 × 10¹² 1 × 10¹²1 × 10⁹  performance (Ω) (after wet heat process)

TABLE 7 Comparative Example 4 (wt parts) Comparative Experi- Experi-Experi- Example 5 Components ment f ment g ment h (wt parts) Benzylacrylate 70 70 70 70 Acryloyl morpholine 30 30 30 30 Polyfunctional 0.50.5 0.5 0.5 monomer Photopolymerization 0.5 0.5 0.5 0.5 initiatorBisphenol A 0 50 250 — bis(diphenyl)phosphate Ammonium — — — 200polyphosphate Flame retardancy NA NA NA V-0 (UL-94) Adhesion (kgf/5 cm)NA NA 0.3 NA Swell Ratio (Times) 9 9 4 — Insulating performance 9 × 10¹³9 × 10¹³ 9 × 10¹³ 3 × 10¹⁴ (Ω) (before wet heat process) Insulatingperformance 2 × 10¹² 2 × 10¹² 2 × 10¹² 3 × 10¹⁰ (Ω) (after wet heatprocess)

As shown by the results of Table 6, the liquid absorbing sheets ofExperiments a through d of Example 16 each had a good flame retardancyand adhesion, swelled 6 times or more, and exhibited a superiorinsulating performance. The liquid absorbing sheet of Experiment e had ahigh flame retardancy, adhesion, and swell ratio comparable to those ofthe liquid absorbing sheets of Experiments a through d. In fact, theliquid absorbing sheet of Experiment e showed higher adhesion than theother liquid absorbing sheets. While the liquid absorbing sheet ofExperiment e had relatively low insulating performance, the fluctuationof the insulating performance was small over the wet heat process,proving high storage stability of the liquid absorbing sheet.

The results of Table 7 for the liquid absorbing sheets of Experiments fthrough h of Comparative Example 4 indicate that the desired flameretardancy may not be achieved not only when bisphenol Abis(diphenyl)phosphate, a phosphate-based liquid flame retardant, is notadded, but also when its amount is too little or too much. The liquidabsorbing sheet of Comparative Example 5, which used the ammoniumpolyphosphate-based solid flame retardant, showed no adhesion, and itsinsulation performance was significantly decreased after the wet heatprocess.

Example 17 Test for the Ability to Absorb an Electrolyte Solution Usinga Simulated Battery Pack

As shown in FIG. 4, a 7.0 cm (1)×7.9 cm (w)×2.3 cm (h) ABS resin box 41was obtained. A 6.5 cm (1)×6.5 cm (w) 100 μm-thick liquid absorbingsheet 42 prepared according to Example 1 was stuck to the bottom of thebox with a commercially available adhesive. Three lithium ion batteries43 were placed on the liquid absorbing sheet 42, and a glass epoxysubstrate 44 as a circuit board was placed adjacent to the batteries.

A hole h was drilled through the side wall of one of the batteries 43that was in the middle of the three. The electrolyte solution leakingfrom the hole was allowed to be absorbed by the liquid absorbing sheet.After drilling of the hole h, the batteries were left for one day andnight and the inside of the battery pack was observed. It turned outthat the glass epoxy substrate 44 was not wet. In addition, the decreasein weight of the battery with the hole h drilled in it was 2.5 g, whichwas equal to the increase in weight of the liquid absorbing sheet. Theseobservations suggest that the leaked electrolyte solution was entirelyabsorbed by the liquid absorbing sheet.

INDUSTRIAL APPLICABILITY

As set forth, the liquid absorbing sheet of the present inventionincludes a liquid absorbing resin layer that can effectively absorb thenonaqueous electrolyte solution used in nonaqueous electrolyte secondarybatteries. This makes the liquid absorbing sheet an effectiveliquid-absorbing element suitable for use in nonaqueous electrolytebattery packs (in particular, in lithium ion-based nonaqueouselectrolyte secondary battery packs).

A phosphate-based liquid flame retardant may be added to the liquidabsorbing resin layer of the liquid absorbing sheet of the presentinvention. When added, the agent imparts superior flame retardancy tothe liquid absorbing sheet. As a result, not only does the liquidabsorbing sheet exhibit the high ability to absorb the nonaqueouselectrolyte solution, which is required in nonaqueous electrolytesecondary batteries to make nonaqueous electrolyte battery packs (inparticular, in lithium ion-based nonaqueous electrolyte secondarybattery packs), but it also shows adhesion and superb flame retardancy.Accordingly, the liquid absorbing sheet of the present invention issuitable as a liquid-absorbing element for absorbing the electrolytesolution in nonaqueous electrolyte battery packs.

1. A liquid absorbing sheet comprising a liquid absorbing resin layer,wherein the liquid absorbing resin layer is obtained by irradiatingUV-rays onto a monomer composition to polymerize the monomercomposition, the monomer composition containing: a monofunctionalmonomer component containing a monofunctional monomer capable of forminga homopolymer that is soluble in a nonaqueous solvent used in anonaqueous electrolyte secondary battery; and a polyfunctional monomercomponent.
 2. The liquid absorbing sheet according to claim 1, whereinthe difference between a solubility parameter value of themonofunctional monomer and a solubility parameter value of thenonaqueous solvent is in the range of −1.0 to 8.0.
 3. The liquidabsorbing sheet according to claim 2, wherein the nonaqueous solvent hasa solubility parameter value of 17 to
 28. 4. The liquid absorbing sheetaccording to claim 1, wherein the nonaqueous solvent contains at leastone of dimethyl carbonate, propylene carbonate, and ethylene carbonate,and the homopolymer is obtained by adding 0.1 to 5 parts by weight of aUV-polymerization initiator per 100 parts by weight of themonofunctional monomer and irradiating UV-rays onto the mixture.
 5. Theliquid absorbing sheet according to claim 4, wherein the homopolymerdissolves in the nonaqueous solvent when 1 part by weight of thehomopolymer is immersed in 30 parts by weight of the mixed solvent atroom temperature for 24 hours.
 6. The liquid absorbing sheet accordingto claim 1, wherein the monofunctional monomer is benzyl acrylate,N-vinyl-2-pyrrolidone, imide acrylate, acryloyl morpholine, phenoxyethylacrylate, N,N-diethylacrylamide, methoxypolyethylene glycol acrylate,tetrahydrofurfuryl acrylate, or phenoxypolyethylene glycol acrylate. 7.The liquid absorbing sheet according to claim 1, wherein the liquidabsorbing resin layer has a crosslink density of 0.0001 to 0.17.
 8. Theliquid absorbing sheet according to claim 1, wherein the liquidabsorbing resin layer is formed on a substrate.
 9. The liquid absorbingsheet according to claim 8, wherein the substrate is capable ofabsorbing and retaining the nonaqueous electrolyte solution.
 10. Theliquid absorbing sheet according to claim 1, wherein the monomercomposition further contains a phosphate-based liquid flame retardant.11. The liquid absorbing sheet according to claim 10, wherein thephosphate-based liquid flame retardant is bisphenol Abis(diphenyl)phosphate, hydroquinol bis(diphenyl)phosphate, phenyldixylenyl phosphate, tricresyl phosphate, cresyl diphenyl phosphate,trixylenyl phosphate, xylenyl diphenyl phosphate, resorcinolbis(diphenyl)phosphate, or 2-ethylhexyl diphenyl phosphate.
 12. Theliquid absorbing sheet according to claim 10, wherein thephosphate-based liquid flame retardant is present in the monomercomposition in an amount of 70 to 200 parts by weight with respect to100 parts by weight of the monofunctional monomer component and thepolyfunctional monomer component combined.
 13. A nonaqueous electrolytebattery pack comprising a nonaqueous electrolyte battery cell, a circuitboard, an electrolyte-absorbing element for absorbing an electrolytesolution in the event of electrolyte leakage from the nonaqueouselectrolyte battery cell, and a battery case encasing the battery cell,the circuit board and the electrolyte-absorbing element, characterizedin that the electrolyte-absorbing element is formed of the liquidabsorbing sheet according to claim 1.